666 ANATOMY AND PHYSIOLOGY OF THE HORSE^S FOOT. 
The groove separating the two projections, and which may 
not improperly be designated the “ perioplic ” or “ coronary 
fissure,” runs parallel with these around the whole circum- 
ference of the foot, and is widest and deepest in front, be- 
coming gradually effaced as it attains the plantar cushion, 
where it is seen to join its extremities on what is called the 
pyramidal portion of that structure. Throughout its whole 
length it is also covered with fine villi or tufts, and from the 
bottom of its groove spring the thin upper and outer margin 
of the hoof. 
The large prominent convex mass beneath the perioplic 
ring (fig. 12, c) has received various designations, according 
to its functions, form, or structure. By some it has been 
called the “ cutis-dura ” from its texture ; “ coronary liga- 
ment ” from its situation and an erroneous estimate of its 
structure ; “ coronary substance ” for want of a more appro- 
priate name, or through ignorance of its function or composi- 
tion ; “ matrix of the hoof ” from its analogy in part of its 
function to that portion of the human digit which secretes the 
nail ; and “ bourrelet,” or ring, by the French, from its 
form. To prevent misconception, however, and to indicate 
one of its most important offices, while harmonising it in 
this respect with a structure already described, I have thought 
it most convenient to designate it by the simple and easily 
remembered name of coronary cushion.” 
It is constituted, as already remarked, by an extraordinary 
thickening of the integument within the upper border of the 
wall of the hoof, and is nothing more or less than a highly 
exaggerated development of the inner layer or derm of the skin. 
It stands out boldly from the parts below like a kind of cornice 
or convex protuberance, most prominent in front, where it is 
so salient as to be conspicuous even to the casual observer. 
From this point it passes backwards from the summit of the 
pyramidal eminence in an oblique manner and in gradually 
diminishing proportions, until it gains the posterior part of 
the foot, or heels, where it bends acutely downwards and 
then forwards, in a manner corresponding with what are 
called the “ bars ” of the hoof, and again attains noticeable 
proportions, but only to grow attenuated within each wing of 
the os pedis, parallel to the space separating it from the 
plantar cushion. At the heels it apparently incorporates 
itself with the cartilaginous bulbs, and through them is allied 
with the plantar cushion. Above, as we have observed, it is 
partly isolated by a somewhat deep circular groove, the 
“ coronary fissure;” but below it is limited by a lighter 
coloured band, the “ coronary” or “ white zone ” (fig. 12, d ), 
